That would break for browsers that understood ::selection and break for Firefox that only understood ::-moz-selection. It made it ripe territory for a preprocessor @mixin, that’s for sure.

That was annoying enough that browsers have apparently fixed it. In a conversation with Estelle Weyl, I learned that this is being changed. She wrote in the MDN docs:

Generally, if there is an invalid pseudo-element or pseudo-class within in a chain or group of selectors, the whole selector list is invalid. If a pseudo-element (but not pseudo-class) has a -webkit- prefix, As of Firefox 63, Blink, Webkit and Gecko browsers assume it is valid, not invalidating the selector list.

This isn’t for any selector; it’s specifically for pseudo-elements. That is, double colons (::).

Here’s a test:

See the Pen Ignored Invalid Selecotrs??? by Chris Coyier (@chriscoyier) on CodePen.